A computer network may for some purposes be usefully described in terms of a physical layer and a logical layer. The physical layer includes the hardware for transmission of information-carrying signals over, for example, electrical cable, fiber optic line or wireless links. This physical layer corresponds to the physical layer, or Layer 1, of the seven-layer Open System Interconnection (OSI) reference model. The logical layer generally includes the protocols used for addressing in the network and routing of information from its source to its destination. The logical layer generally corresponds to the data link layer (Layer 2) and/or the network layer (Layer 3) of the OSI model. Part of the routing function of the logical layer in a computer network is re-routing to avoid disruption caused by a network failure, or to recover from such a disruption. The re-routing involves identifying a backup path for the information being routed. The feasibility of a backup path, however, may depend upon the reason for the network failure. If the failure of the original routing path is due to a broken fiber optic cable, for example, a backup path using the same cable will naturally fail as well. The logical layer routing algorithms are generally unaware of the correspondence between logical network nodes and their underlying physical equipment, and in fact both physical and logical network configurations may be continually changing.
The concept of a shared risk group (SRG) has developed to assist routing algorithms in finding backup paths less likely to be affected by the same network failure afflicting the corresponding primary path. An SRG may also be called a shared risk resource group (SRRG), and is generally a group of routes or paths all of which include a particular network resource, such that all of the paths in the group share the risk presented by a failure of the underlying resource. More specific types of SRG have been defined according to the type of network resource in question, including shared risk link group (SRLG), shared risk node group (SRNG), and shared risk equipment group (SREG). An SRLG is a group of routes having a common link, such as a span of optical fiber. Similarly, an SRNG is a group of routes traversing the same node, and the routes in an SREG share an equipment resource, such as a multiplexer in a wavelength division multiplexed network. “SRG” as used herein may refer to any of these types of shared risk groups. (In some other contexts, including the generalized multi-protocol label switching (“GMPLS”) architecture, “SRLG” may be used to refer to shared risk groups in general.) Because an SRG can be associated with any network resource shared by multiple network routes, a given route may be a member of multiple SRGs. With information on the SRGs associated with potential routes, a routing algorithm can attempt to identify a backup path that is not in the same SRGs as its corresponding primary path; such a backup path would not share the same network resources as the primary path and therefore not be affected by the same network failures.
Association of SRGs with specific network resources is typically done within a particular routing domain. A routing domain as used herein refers to a collection of interconnected network nodes under a common administration for purposes of network configuration. A routing domain may also be referred to as an “autonomous system” (AS). A large network may include many routing domains at any of multiple network levels, each configured by a corresponding individual or organization. (Networks that come under common ownership or administration through, for example, a corporate acquisition process may continue to operate as separate routing domains based on the way they were originally configured.) SRG identifiers may be assigned manually by an administrator for the routing domain, or may in some cases be automatically assigned by network node equipment within the domain.